An aluminum nitride sintered body is suitable for use as a material for an IC package or a semiconductor substrate, due to its characteristics such as high theoretical thermal conductivity, excellent electric insulation, etc.
The thermal conductivity of such an aluminum nitride sintered body is determined by amounts of oxygen, carbon, cations etc. which are solidly dissolved in aluminum nitride crystal grains, as described in "J. Phys. Chem. Solids" Vol. 48, No. 7, 1987, pp. 641-647, for example. Thus, it is extremely important to measure such amounts of dissolved substances for quality assurance of an aluminum nitride sintered body having high thermal conductivity. Further, the thermal conductivity of an aluminum nitride sintered body is remarkably influenced by the oxygen content in aluminum nitride crystal grains forming the aluminum nitride raw material powder. Therefore, it is extremely important to measure the oxygen content in the aluminum nitride powder. However, no method of measuring the oxygen content in aluminum nitride powder or a sintered body thereof has been established heretofore.
For example, measurement of an oxygen content by chemical analysis is merely directed to the total amount of impurities of a tested sample. Such impurities include not only the oxygen contained in aluminum nitride particles of an aluminum nitride powder or a sintered body thereof, but also those impurities adhering to the surface or being contained in a grain boundary phase. By using such a chemical analysis, therefore, it is impossible to selectively measure and evaluate only an impurity which is contained in the aluminum nitride crystal grains, i.e. that impurity which has been noted to remarkably influence thermal conductivity.
There is a method of obtaining an oxygen content value by converting an aluminum nitride lattice constant which is measured by X-ray diffraction. When impurities other than oxygen are solidly dissolved in the aluminum nitride crystal grains, however, the lattice constant is not in one-to-one correspondence with the impurity content such as the oxygen content. Thus, this method is also unsuitable for evaluation of aluminum nitride powder or a sintered body thereof.
Measuring an impurity content such as an oxygen content by a surface analysis method such as Auger electron spectroscopy or secondary ion mass spectroscopy is not suitable for the following reasons. First, it is impossible to measure the impurity content if the aluminum nitride crystals contain the impurity in an amount below the detection sensitivity of the measuring method. Second, when the crystals contain the impurity in an amount exceeding the detection sensitivity, on the other hand, impurities which have adhered to a surface of the sample during preparation thereof are also inevitably measured together with those contained in the crystals. Thus, this method is not suitable because of its lack of accuracy in measurement. Third, the amount of time required for the measurement is excessive when the impurity content is measured over a depth of several nm. Fourth, the surface analysis method is directed only to a local region, and is therefore not applicable to evaluation of the overall product.
As described above, it is impossible to detect and measure an impurity such as oxygen contained in the aluminum nitride crystal grains forming an aluminum nitride powder or a sintered body thereof, in a short time over the entire product using the conventional measuring apparatus and method.
Further, quality assurance evaluation as to the thermal conductivity of an aluminum nitride sintered body inevitably depends on the measurement of thermal conductivity by a laser flash method or the like. When using a method of directly measuring thermal conductivity such as the laser flash method, however, a sample for measurement is restricted in size and shape. For example, it is impossible to directly measure thermal conductivity of a product having a thickness of 0.5 mm and a width of 2 to 3 mm. Thus, it is necessary to independently prepare a measurable sample for measuring its thermal conductivity, thereby assuring quality of the product. This is a factor which increases the manufacturing cost. In order to evaluate the quality of a product having a complicated shape, furthermore, a sample is partially cut out to be subjected to measurement of thermal conductivity, thereby carrying out quality evaluation of the product. Thus, it is impossible to evaluate the quality of the overall or entire product.